1. Field of the Invention
This invention relates to mixtures of organosilane-polysulphanes having an elevated proportion of disulphanes and to a process for the production of rubber compounds containing these compounds.
2. Prior Art
Due to increasing environmental awareness, economies in fuel consumption and a reduction in pollutant emissions are today becoming a greater priority [Auto 91/92, Verband der Autombobilindustrie e.V., Frankfurt, ADAC-Motorwelt 11791, 50 (1991]. The challenge to tire manufacturers is to develop tires which are distinguished by very low rolling resistance combined with excellent wet skid resistance and good abrasion resistance.
Proposals have been made in numerous publications and patents with regard to reducing tire rolling resistance and thus fuel consumption. Proposals which may be mentioned include reducing the carbon black content in the compound and using special carbon blacks (U.S. Pat. No. 4,866,131, U.S. Pat. No. 4,894,420). However, none of these proposed solutions has resulted in a satisfactory balance between the desired low rolling resistance and other important tire characteristics such as wet skid resistance and abrasion resistance.
The use of highly active silica fillers in combination with organosilane bis(triethoxysilylpropyl)tetrasulphane (TESPT) to replace the carbon black normally used in rubber compounds is known to allow production of a tire having rolling resistance substantially reduced in comparison with standard tires, while simultaneously retaining or even improving the other two above-stated tire characteristics [EP 0 501 227, U.S. Pat. No. 5,227,425; G. Agostini, J. Berg, Th. Materne: New Compound Technology, October 1994, Akron, Ohio, USA; S. Wolff, U. Gorl, M. J. Wang, W. Wolff: Silica based on tread compounds--background & performance, paper presented at TYRE TECH '93, October 1993, Basel, Switzerland; Ph. Cochet, L. B. Barriquand: Precipitated silica in tire tread, paper presented at ACS Meeting of the Rubber Division, October 1995, Cleveland, Ohio, USA].
At the 1986 ACS meeting in New York S. Wolff [S. Wolff: The influence of fillers on rolling resistance, presented at the 129.sup.th meeting of the Rubber Division, American Chemical Society, Apr. 8-11, 1986, New York] presented a paper showing that it is possible to reduce rolling resistance in comparison with a carbon black filled standard compound while largely retaining wet skid resistance by using silica in combination with TESPT both in a passenger vehicle tire tread based on an emulsion styrene/butadiene rubber (E-SBR) and in a truck tire tread based on natural rubber.
This system was further optimized with regard to all three characteristics by using specific styrene/butadiene polymers produced using a solution polymerization process (EP 0 447 066 A1), sometimes blended with other polymers, in particular polybutadiene and additionally using novel grades of silica (U.S. Pat. No. 5,227,425) and polymer blends specifically tailored to this application (EP 0 620 250 A1) sometimes with three to four different starting polymers [G. W. Marwede, U. G. Eisele, A. J. M. Sumner: paper presented to the ACS Meeting of the Rubber Division, October 1995, Cleveland, Ohio, USA.].
It is stated in all the publications and patents that, in order to achieve a lower rolling resistance while retaining or improving wet skid resistance and abrasion resistance, it is necessary to replace a large proportion of or the entire content of the normally used carbon black filler with a highly active silica [S. Wolff: The influence of fillers on rolling resistance, presented at the 129.sup.th meeting of the Rubber Division, America Chemical Society, Apr. 8-11, 1986, New York; U. LeMaitre: Tire rolling resistance, AFCEP/DKG Meeting, 1993, Mulhouse, France]. However, this replacement results in the desired objective only if the organosilane bis(triethoxysilylpropyl)tetrasulphane (TESTP) is used as a "coupling" agent between the silica and the polymer.
It is known [S. Wolff: The role of rubber-to-silica bonds in reinforcement, presented at the First Franco-German Rubber Symposium, Nov. 14-16, 1985, Obernai, France; S. Wolff: Silanes in tire compounding after ten years--review, Third Annual Meeting & Conference on Tire Science & Technology, The Tire Society, Mar. 28-29, 1984, Akron, Ohio, USA] that the properties which may be achieved by using organosilanes in rubber compounds are dependent upon two independent reactions. Firstly, during production of the compound, preferably during the first compounding stage, a reaction occurs at elevated temperature between the silanol groups of the silica and the trialkoxysilyl groups of the silane with elimination of alcohol (hydrophobing or modification reaction). A complete reaction is of decisive significance to subsequent properties.
Like all chemical reactions, this reaction proceeds faster at elevated temperatures [U. Gorl, A. Hunsche: Advanced investigations into the silica/silane reaction system, paper presented at ACS Meeting, Rubber Division, October 1996, Louisville, Ky., USA], such that the rubber compounder, desiring short compounding times, prefers to use the highest possible compounding temperature. The use of such high compounding temperatures is, however, limited by the fact that the second, so-called rubber-reactive group of TESPT consists of a group which is on average a tetrasulphane group having a significant proportion of longer sulphane chains (S.sub.5 -S.sub.8) [S. Wolff: Silanes in tire compounding after ten years--review, Third Annual Meeting & Conference on Tire Science & Technology, The Tire Society, Mar. 28-29, 1984, Akron, Ohio, USA].
This rubber-reactive group is generally considered to give rise to a so-called filler/rubber bond, which determines the technical rubber properties of the finished article (for example tires). This reaction, which is desired during vulcanization, is influenced by the thermal lability of the tetrasulphane group and higher sulphane units. Practical experience has, however, shown that the reaction causes serious problems if it occurs during production of the unvulcanised compound, during which only the reaction between the filler and the silane should normally occur.
If sulphur is eliminated from the long-chain sulphane units, it is incorporated into the polymer chain. This then brings about "scorching" and stiffening of the sheeted compound, which can even render the unvulcanized compound unprocessable. Scorching can be measured by determining the viscosity of the compound. EP-A1-0 732 362, which is not a prior publication, describes the use of organosilanedisulphides in rubber compounds.
However, these sulphur compounds must be very pure or have a disulphide content of at least 80%.